Bareback means riding a horse with no saddle. Ride bareback, bareback riding, bareback across the field.
It is harder and closer at once: without a saddle the rider grips with the legs and feels every movement, which is why it is used in teaching seat and balance. Circus bareback riding is a discipline of its own. Note that the word is simply bare plus back, and that it works as both adjective and adverb.
1adverbbarebackRiding a horse with no saddle.
“The children rode bareback along the beach, holding on to the horses' manes.”
bareback is a way of riding.
Bare plus back, a plain English compound describing the horse rather than the rider.
From bare + back.
Etymology from Wiktionary, CC BY-SA